The present invention relates to a diaphragm value signal generator for photographic cameras of automatic diaphragm control type.
As a leading trend in light measuring techniques for modern photographic cameras, there has commonly been adopted a so-called full aperture metering technique by which the light coming from an object to be photographed through the objective with fully opened aperture is received within the camera interior to perform light measuring, because of various advantages such as a bright visual field and easy focusing. In this full aperture metering technique, conventionally luminance of the light transmitted through the objective with fully opened aperture, preset shutter speed, sensitivity of the film being used, and, if desired by the photographer, a selectively preset exposure multiple are subjected to arithmetic processing to determine an appropriate calculated diaphragm value and, after shutter release, the diaphragm is mechanically moved, i.e., stopped down, until said calculated diaphragm value is attained. With this technique, furthermore, it has been required that the actual diaphragm valve is continuously monitored in the course of stopping down of the diaphragm and the diaphragm movement is arrested and fixed at the moment when the actual diaphragm value coincides with the calculated diaphragm value.
For such monitoring of the diaphragm movement, there has usually been provided a pulse generator adapted to emit electric pulses in response to movement of a member within the camera body and representative of the diaphragm's stopping down movement so that the number of pulses emitted by the pulse generator depends on the amount of stop down of the diaphragm. These pulses are counted by a counter and the accumulated count value represents the actual diaphragm value. Details of such a mechanism are well known from, for example, application Ser. No. 258,044, filed Apr. 28, 1981, the disclosure of which is incorporated fully herein by reference. However, this is a technique in which the actual diaphragm value is indirectly determined by monitoring the amount of movement of the diaphragm driving member, so that a certain amount of error is inevitably included in the detected diaphragm value signal due to various causes such as play inherent in the mechanical linkages connected between the diaphragm driving member and the diaphragm blades, and the time delay caused by load torque in transmission of diaphragm value. Such error varies, particularly in photographic cameras using many interchangeable lenses, and differs for variously mechanical transmission linkages. In consequence, the camera often causes false exposure.